1. Field of the Invention
The present invention relates to an image synthesis apparatus that synthesizes a plurality of images by layering the images to generate a new image. Such a technique of layering images to synthesize them to generate a new image is generally referred to as a “layer synthesis” or an “overlay”.
2. Description of the Related Art
Image synthesis apparatuses that synthesize a plurality of images according to layer synthesis to generate a new image exist conventionally.
Here, layer synthesis refers to synthesizing a plurality of images by layering them on each other in a predetermined order. This image synthesis method used, for instance, when superimposing an image or images on a background image.
In a general layer synthesis apparatus, a plurality of images to be used in synthesis (source images) are read in order from a memory, layered one at a time, and a final synthesized image is output to a display monitor (see Japanese patent No. 3380859, for instance).
Take for example a case in which two images (1) and (2) are synthesized to obtain a synthesized image (5). First, the image (1) is read from the memory and stored in a buffer. Next, the image (2) is read from the memory, and synthesized in a synthesis circuit with the image (1) that is stored in the buffer. This generates the synthesized image (5) which, after being stored in the memory, is output to the display monitor.
In a case in which four images (1) to (4) are synthesized to obtain a synthesized image (6), first the image (1) stored in the buffer and the image (2) subsequently read from the memory are synthesized with each other in the synthesis circuit, and a generated synthesized image (1)+(2) is stored in the buffer. Next, the image (3) is read from the memory, and synthesized with the synthesized image (1)+(2) to generate a synthesized image (1)+(2)+(3) which is then stored in the buffer. Finally, the remaining image (4) is read from the memory and synthesized with the synthesized image (1)+(2)+(3) stored in the buffer. This generates the synthesized image (6) which, after being stored in the memory, is output to the display monitor.
However, since this kind of image synthesis apparatus reads each of the plurality of input images that are the source images from the memory and then generates one new image, when two synthesized images are to be generated, the image synthesis apparatus generates the two by performing image synthesis separately for each synthesized image, even if the two synthesized images share common source images.
In other words, taking for instance the aforementioned examples, the synthesized image (5) is composed of source images (1) and (2), and the synthesized image (6) is composed of the source images (1) to (4). Despite the synthesized images (5) and (6) having the source images (1) and (2) in common, the synthesized image (5) is generated by synthesizing the source images (1) and (2) and then output to the memory, while the synthesized image (6) is generated by synthesizing the source images (1), (2), (3) and (4) and then output to the memory.
In this way, a considerable number of memory accesses and layer synthesis processes take place in a conventional image synthesis apparatus because layer synthesis is performed by reading source images one at a time from the memory even if the synthesized images have common source images. In particular, when a relatively large number of synthesized images is to be generated, the image synthesis apparatus incurs a heavy load because the number of memory accesses and the number of times layer synthesis is performed increases correspondingly. This means that a considerable amount of time is taken to generate the two synthesized images.
In view of this problem, the present invention has an object of providing an image synthesis apparatus and method that reduce the load incurred when generating a plurality of synthesized images according to layer synthesis.